There are numerous theses about the origin of music. The following are some of the more common ones: 1) It emerges as part of sexual selection. Just as the Bowerbirds seduce by building colorful nests, or bird song allures, humans developed art and music as part of a biological phenomenon of sexual selection. Where long flowing hair, bespeaking fertility, might not have been enough, the creation of artifacts or pleasing tones further served sexual selection. 2) It originates prior to or along with language development, facilitating it. Tolstoy had already pointed to an artistic expression existing as long as linguistic expression. More recently, many ethnomusicologists and evolutionary theorists have maintained that music even facilitated language development. Vocalizations in pre-human animals served many purposes, for example, to indicate to members of one’s group that one was still within distance or to indicate displeasure or pleasure at some state of affairs, often here doing what it is so known for, namely conveying emotions. The short version of this story is that the rhythmic and tonal developments expressing emotion were co-developed with or facilitated the development of language itself, where specific tones came to symbolize objects in the world or perceived states of affairs. 3) It develops as an aid to early childrearing. Lullabies are known in all cultures. Music may be emerged as a tool to soothe and comfort infants. 4) It emerges as a tool to facilitate social coordination. Throughout human society, we see music used to coordinate collective work of individuals in society. Think of music used in prison chain gangs as one vivid example. But the facilitation of social coordination is broader: For example, we see music used in social groups — in modern incarnations from national anthems to the Internationale. Music builds bonds. It facilitates identity formation. We see this in diverse social formations (even as ethnomusicology studies the ethnic music of diverse peoples or as music defines identities of subcultures).
More recently, 5) neurologists, like Oliver Sachs, have pointed to our music ability as fundamental to our bodily rhythmic coordination. It has also long been seen as 6) facilitating a human experience of transcendence. From early in human history, music has played a role in religion and spiritual experience, as it reinforces the individuals’ sense that they part of a larger spiritual whole. We see this around the world today in chants of Buddhist and Cistercian monks, in Sufi music and dance, in voodoo drumming. Related to point four above, about social coordination and identity, it also facilitates less traditionally spiritual senses of transcendence. Through art and music individuals transcend their own narrowly egoist identities (insofar as they ever have these at all) and further construct their identities as part of social groups or generations.
One need not accept a monocausal view — that music emerged for any one of these reasons alone. Perhaps it served many of these purposes from the outset. It was advantageous for humans for multiple reasons and music making has arisen as a universal characteristic of human cultural life because it has continued to facilitate multiple such purposes. In many of these ways mentioned, music allows the expression and cultivation of emotion and is able then to be used to channel emotion toward diverse ends. Mixed with language, it also facilitates story telling in cultures. It gets taken up in dramatology and more.
Though I won’t take up the arguments that music making is a simple add on to language, as Steven Pinker suggests, the evidence seems less and less to support this. Tolstoy was likely more correct that art (and we can add music) is as old as language itself. Music is one medium of human, even rational, expression that has multiple uses for the individual and society.
On the view that music, in this process, is in part at least also a rational form of human expression, see the following posts on music and reason and thoughts without words.